Generally, liquid developers for electrostatic photography are obtained by uniformly dispersing inorganic or organic pigments or dyes such as carbon black, nigrosine and phthalocyanine blue and natural or synthetic resins such as alkyd resins, acrylic resins, rosin and synthetic rubbers in liquids having high insulating properties and low dielectric constants, such as petroleum-type aliphatic hydrocarbons, and further adding metal soaps, lecithin, linseed oil, higher fatty acids, a polymer containing vinylpyrrolidone, etc. for the purpose of making the charge characteristics of the dispersed particles uniform and strong.
In a developing process, such a developer undergoes electrophoresis according to the charge of the electrostatic latent image formed on the surface layer of the electrophotographic material or the electrostatic recording material and becomes fixed to that surface to form a visible image (copied image). However, the conventional liquid developers are disadvantageous in that a dispersion stabilizing resin (i.e., dispersant) or a charge controlling agent diffuses in the liquid to obscure the charge characteristics. As a result, the copied images lose clarity due to the reduction of image density and fixation, the increase of background fogging, etc.
In addition, these developers are susceptible to sedimentation, flocculation, etc. of the dispersed particles over the course of time. Once these particles have been sedimented or flocculated, they cannot be redispersed. Hence, it is difficult to use such developers.
Furthermore, because of these disadvantages, conventional liquid developers are also unsuitable for use in offset printing, or for transfer purposes, for example charge transfer, press transfer or magnetic transfer.
As a method of overcoming these problems, it has been suggested to stabilize the dispersion of the particles by grafting a polymer to the surface of a pigment such as carbon black. However, a developer obtained by using this method has the disadvantage that the relative amount of the resinous component which adheres to the image surface together with the pigment is small, and the strength of the formed image after fixation is insufficient.
Accordingly, when an image is formed on a zinc oxide photosensitive sheet using such developer and the sheet is used as an offset printing plate, the hydrophobic property of the printing plate to printing inks and the number of printed copies which can be produced thereby are insufficient due to the aforesaid causes.
U.S. Pat. No. 3,990,980 discloses another method in which monomers are polymerized in the aforesaid non-aqueous solvent in the presence of a dispersant having a grafting group to form fine resin particles, and these resin particles are used as toner particles. However, the use of such a liquid developer still causes problems. Specifically, the dispersion stability of this developer against spontaneous sedimentation is improved to some extent, but still remains insufficient. When such a developer is used in a developing device, the toner adhering to the various parts of the device solidifies in film form, and it is difficult to redisperse them. Furthermore, the use of such a developer causes problems in the device itself, the stain of copied images or etc. Moreover, there is a marked restriction on the combination of dispersant having a grafting group and monomer employed in order to form satisfactory resin particles by the above method. Further, generally, the resulting resin particles contain large amounts of coarse particles and have a broad particle size distribution. Another problem is that desirable dispersant have to be synthesized through complex manufacturing steps.